So-called “section stretchers,” or devices for stretching sample sections, are known from the existing art of microtomes. Microtomes serve for the production of thin sections of various prepared samples, such as tissue specimens, in medicine or biology. Such samples either can be sectioned while frozen, or are embedded into an embedding medium (usually paraffin) and sectioned together with it. Extremely sharp knives that are secured in a knife holder are used to section the samples. The thickness of the thin sections is generally in the micrometer range, while typical lengths and widths are in the range from 5 mm to 30 mm. The sample sections are usually mounted onto a specimen slide so they can then be investigated microscopically. The sections tend to roll up upon sectioning. Rolled-up sample sections are, however, very difficult to mount onto a specimen slide, and the risk also exists that the rolled-up sample sections may break. The risk of breakage also exists if a manual attempt is made to grasp the section with a brush or a forceps and pull it away from the knife blade in order to prevent rolling.
So-called “section stretchers,” or section stretching devices, are used in order to prevent the sample sections from undesirably rolling up. Section stretchers comprise a glass plate or Plexiglas plate that is arranged at the knife back, parallel to and at a short distance from the knife blade, so that a thin gap is produced between the knife back and the plate. The “knife back” is understood as that part of the knife which is adjacent to the knife blade (also called the “edge” or “bevel”) and slides over the sample section before it detaches from the sample. During sectioning, the sectioned sample thus pushes into the gap between the glass plate or Plexiglas plate and the aforesaid knife back.
After sectioning, the section stretcher is removed from the knife back and the sectioned sample is removed. In the case of cryostat sectioning, this removal is effected by the fact that a specimen slide warmed to room temperature is placed against the knife edge and slowly lowered onto the sectioned sample. Upon contact, the ice in the sample abruptly melts and the sectioned sample remains adhered onto the specimen slide.
DE 100 48 724 B4 discloses a device for stretching cryostat sections, having a section stretcher described above. The gap between the plate and knife back is typically 0.15 mm. In this document, this gap is embodied in defined fashion by the fact that the glass plate or Plexiglas plate is mounted in a frame that comprises internally shaped-on struts. The struts are equipped with a support surface for the plate. The frame further comprises an abutment edge for placement of the frame onto the back surface of the sectioning knife. The spacing between said abutment edge and the support surface for the plate then forms the defined gap for reception of the sectioned sample, which is effectively prevented from rolling up. A further result of this embodiment of the section stretcher is that no further contact occurs between the sensitive plate and the sectioning knife. The dimension of the gap is furthermore no longer dependent on the material thicknesses of the plates, but instead is defined unequivocally by the spacings on the frame. Plates of different material thicknesses can thus also be used, with no change in the dimension of the gap.
With regard to further details of the manner of operation and the configuration of the section stretcher or the corresponding device for stretching cryostat sections, reference may be made explicitly to the aforementioned DE 100 48 724 B4 in its entirety. For purposes of disclosure, the content of this patent is assumed to be incorporated into the present application.
Another device for stretching sample sections is known from DE 20 2010 011 369 U1. Here the section stretcher serves at the same time as a finger protector for an operator who is manually removing the sample section from the knife back. What is proposed is a part or member, triangular in section and extending over the length of the knife blade, that is arranged at a specific distance from the sample and at a specific distance from the knife back. The depth of the gap existing with respect to the knife back is dimensioned in such a way that rolling of the section is prevented, but on the other hand the depth of the gap is at most such that sections of a usual dimension emerge from the gap, in the course of the sectioning motion, with their front side in the flow direction. The height of the gap here is typically between 0.15 mm and 0.25 mm, while the depth of the gap is between 3 mm and 5 mm.
Another type of section stretcher is known from DE 27 32 001 C2. Here the section stretcher is retained by magnetic force, which is said to ensure unequivocal and reproducible retention as well as consistent gap widths.
DE 100 13 693 B4 relates to a method for manufacturing a section stretcher plate from inorganic glass. This document mentions the previous practice of grinding and polishing the edge lines of a glass plate that were obtained by bending fracture. As stated in this document, however, the high edge quality of the edge lines would be affected by the grinding and polishing. To prevent this, the teaching of this document provides for controlled rounding in the course of a subsequent thermal or chemical step. In thermal rounding, for example, each edge line is processed with a CO2 laser, with the result that the edge line is slightly melted and edge rounding occurs. In chemical rounding, the edge line to be rounded is immersed into a chemical bath in which, for example, potassium ions are dissolved. Hardening is simultaneously achieved thereby.
GB 1,058,696 A discloses coating a section stretching glass plate with a plastic material such as PTFE that is sintered onto the surface of the glass plate.
Lastly, DE 100 13 688 A1 discloses a method for manufacturing a section stretcher plate made of inorganic glass, in which the section stretcher plate is detached from a glass surface substrate, by means of stresses thermally induced in that glass surface substrate, along a detachment line that corresponds to the lateral edge of the section stretcher plate.
It has become apparent that in a device for stretching cryostat sections in accordance with DE 100 48 724 B4 already discussed, it is advantageous to use glass plates in the section stretcher, since glass plates, in contrast to Plexiglas plates, scratch less easily and do not acquire an appreciable static charge when used. Chemical resistance (for example in the context of cleaning, or upon contact with the sample section) is also greater, so that glass plates less quickly become opaque or “blind”. It has furthermore become apparent that sample sections can get caught, or become shredded, on the edges of the glass plates.
An object of the present invention is therefore to optimize glass plates used in section stretchers of the kind recited previously in order to eliminate the aforementioned disadvantages in the context of the sectioning of samples.